The invention relates to processes which are used to deposit thin films of materials by processes such as chemical vapor deposition (CVD), in which chemically reactive gaseous species are introduced into the processing environment under controlled conditions of temperatures, gas flow and pressure, and in some cases additional plasma or optical excitation to cause the deposition of desired materials in thin film form on a substrate surface such as a semiconductor wafer. The deposition occurs because of chemical reactions between the gaseous species, usually involving reactions on the surface where deposition is desired, but sometimes involving reactions which occur in the gas phase and lead to formation of new species which then deposit on the surface.
CVD is a widely used unit operation in the semiconductor manufacturing industry for thin film device production. The continuing reduction of device feature size and the development of new microelectronic devices have increased the demand for new electronic materials which meet specific materials performance objectives. Common modes of operation include (1) thermal CVD, in which the reaction requires only thermal energy (heating) to proceed, (2) plasma CVD, where a plasma discharge in the gas phase promotes the deposition reaction, and (3) others such as photo-CVD, where the deposition reaction is stimulated by optical excitation. To obtain the desired properties in the deposited thin films of metal, insulator, or semiconductor materials, various combinations of gases and process parameters are required. While selecting these optimal combinations has long been a challenge, to meet the demands of feature size reduction and device performance, the challenge is even greater today as fundamentally new and more complex materials and CVD chemistries are required, both for active devices (e.g., high dielectric constant insulators for FET gates) and for advanced interconnections (including low dielectric constant insulators, copper metallurgy, and metal nitride diffusion barrier layers).
Competitive manufacturing of semiconductors imposes a major additional requirement on CVD processes in the form of manufacturing performance. Silicon wafer sizes are being increased from eight inches to twelve inches diameter in order to reduce the cost per chip, where chips are of order 1 cm2 area each. This increase in wafer size means more than twice the number of chips are now produced per wafer processed. However, the properties of each chip on the wafer must be virtually identical, requiring each process to exhibit uniformity of its metrics across the wafer, e.g., to within 1%. Furthermore, the processing rates must be sufficiently high for rapid deposition and high throughput, as needed for cost minimization. Similar considerations apply in other CVD application areas, such as plasma CVD processing of flat panel displays. Besides plasma-enhanced CVD, plasma etch processes are widely exploited for etching of materials, especially directional etching as needed for the fabrication of submicron device and interconnect structures and present similar problems and consideration.
The various CVD and plasma process modes, described above, have been and are regularly employed in the manufacturing of advanced products. Where the products entail a large area, as in the case of large silicon wafers for semiconductor chips or large glass panels for flat panel displays, the materials performance requirements must be met across a wide spatial extent (8-15 inches) and specified spatial uniformity demands for manufacturing performance.
The conventional approach to achieving simultaneous materials performance and across-wafer uniformity for manufacturing is to design the CVD equipment for single-wafer processing so that gas fluxes impinge as uniformly as possible across the wafer. To attempt to obtain maximum uniformity of gas impingement, some or all of the gases are delivered to the wafer through a showerhead, consisting of a flat plate parallel to and near the wafer surface. The gas passes through a high density of uniformly spaced small holes in the showerhead, thus distributing the gas flow as uniformly as possible across a large diameter wafer. In addition, reactor design componentsxe2x80x94including chamber, wafer position (and rotation), pumping, heating, and gas inletxe2x80x94are constructed to attempt to maximize uniformity in terms of 2-D cylindrical symmetry about the wafer.
Various showerhead designs have been developed to attempt to generate uniform gas flow patterns over the wafer surface or for uniform film deposition. The requirement of across-wafer process uniformity has been a major driving force for the industry trend to single-wafer processing and the delivery of gases through a showerhead in relatively close proximity to the wafer surface (from about 2 to 20 mm).
For plasma processing equipment, the power distribution means used to generate the plasma must also be designed to attempt to produce uniform effects across the wafer. Much of plasma processing equipment today is single-wafer. For reactive ion etching and for plasma CVD, gas is introduced through a showerhead parallel to and near the wafer surface. This showerhead serves to distribute the reactant gas species in a relatively uniform manner and also as a counterelectrode for the plasma discharge, with the wafer attached to the other electrode.
Known single-wafer CVD and plasma process equipment using showerhead gas delivery provides a reasonably high degree of symmetry to the process. However, because the gas is introduced as uniform flux but is pumped away at the edges of the wafer, the deposition symmetry is radial, so that nonuniformities are experienced primarily in the radial direction, e.g., thicker films result in the wafer center region relative to the edges. Because the deposition reaction consumes the impinging reactant species, the flow of gases radially across the wafer leads to radial nonuniformities, the extent of which depend on the particular chemical species in use.
A more flexible design to achieve increased radial uniformity for complex CVD chemistries involves a three-zone showerhead as disclosed in U.S. Pat. No. 5,453,124 to Moslehi et al. which has been used for tungsten CVD. In this system, gas is introduced from three independently controlled concentric annular rings, each of which features individual zone feed gas mass flow controllers with potential for real-time control of process gas flows to each annular segment. The center region is circular, while the outer two are doughnut-shaped. By changing the gas flows in one zone relative to another, one can attempt to alter radial profile of deposition rate.
In practice, this has seen limited use for depositing metal compound barrier layers, using a single feed gas and with manually switched flow conductance elements to shorten development cycle time for new process equipment. Although this design has been able to achieve some improved radial uniformity, it still presents significant drawbacks in that it presents a single fixed rather than modular construction, it does not provide for exhaust gas sampling through or real-time sensing in the showerhead, it only permits control of processed gas flows to fixed annular segments, and due to the fact that the gas is pumped away at the edges of the wafer, significant intersegment convective mixing occurs.
Other approaches to controlling process uniformity have been directed to attempting to control spatial distribution of process variables other than gas flow. In rapid thermal processing (RTP), wafers are heated rapidly to reaction temperatures and maintained at these temperatures briefly to accomplish annealing, thermal oxidation, or CVD. In RTP, the key issue is temperature uniformity, both during the reaction and during temperature ramp-up. To compensate for radial temperature nonuniformities during RTP (associated primarily with different heat loss rates at the wafer edge cf. its center), multizone lamp heating arrays have been employed. Radial nonuniformities present a problem in plasma processes as well. Radially symmetric, tunable electrode elements such as those disclosed in U.S. Pat. No. 5,716,486 have been proposed to control both process uniformity and the steady-state particle traps which are formed in plasma processes. In all these cases, the equipment design advances have addressed the compensation of only radial nonuniformities.
Despite prior advances, CVD and plasma processes continue to face a major challenge in achieving uniformity of thin film layers and microstructures across the wafer. The first problem is to achieve such uniformity in the product (silicon wafer, flat panel display, etc.) by appropriate design and operation of the processing equipment, so that desired product performance is attained simultaneously with the uniformity needed for efficient and economical manufacturing. This problem is exacerbated not only by the continuous reduction of microfeature sizes to be manufactured on substrates (e.g., wafers, panels) of increasing overall size, but also by important technology trends and manufacturing considerations in the industry.
One such trend is the prominence of new materials and processes in the industry. For silicon chips, ultrasmall devices now require insulators with dielectric constants much larger than that of conventional silicon dioxide. The solutions now being widely pursued are complex multicomponent materials such as barium strontium titanate, strontium bismuth titanate, or tantalum oxide, materials which may require doping as well. These materials require CVD processes for manufacturability, but their intrinsic complexity (three to five chemical elements) exacerbates the challenge in both process development and manufacturability. For interconnection technology, low dielectric constant materials are being heavily pursued, in part through CVD processes, with similar challenges, along with new materials (metal nitrides for barrier layers, copper for wiring) for the metallic components. The materials complexity involved in deposition reappears in the challenge of etching these materials using plasma processes.
Another trend is the difficulty in co-optimizing materials and manufacturing performance since they often present competing considerations. Given a process chemistry, the design point which is best for materials performance may yield poor uniformity in a specific reactor (or indeed in most or all reactor configurations), while process parameters which achieve high uniformity may produce poor materials performance. Hence it is a common problem that materials performance must be compromised to achieve acceptable manufacturing performance (uniformity). Another trend is the escalating cost of manufacturing process equipment, which now dominates the cost of manufacturing facilities.
In the face of this, it is crucial to use the equipment as efficiently as possible, and in particular to minimize the time in which the equipment must be dedicated to testing process development and refinement as opposed to production of completed products. However, the challenge of new materials places an even heavier burden on experimentation to identify suitable process parameters and recipes to use these new materials. Given these strongly competing considerations, rapid materials and process development, therefore, is increasingly important from a cost perspective. In addition, enterprise costs escalate because the lifetime of equipment is limited to only one or two technology generations since they can be readily or economically be modified after the time an entire new equipment design cycle must be carried out and underwritten.
The use of spatially-programmable process parameters within equipment design for CVD and plasma processes has the ability to significantly improve this situation because spatially-tunable process parameters could be exploited to assure uniformity over a wide range of nominal process design points. In particular, multizone showerheads can ensure that uniformity is obtained at CVD or plasma process conditions desired. However, as embodied in prior known multizone showerheads, several important problems have not yet been solved, or explicitly recognized.
First, interzone mixing sharply diminishes the spatial control which is achievable. For example, the three-zone CVD showerhead design disclosed in U.S. Pat. No. 5,453,124 involves the flow of gas from the wafer center across the outer regions of the wafer. As a result, the impinging fluxes in the outer wafer radial positions are directly influenced by the extent of reaction and the impinging gas flow at the center of the wafer. This mixing also has the effect of reducing the resolution capabilities of gas composition sensing techniques that rely on gas sampling at discrete locations in the gas phase.
Second, spatial programmability of the process is only accomplished in the radial direction. In reality, other sources cause non-radial nonuniformities as well, from the asymmetries of gas flow due to upstream and downstream equipment geometry, to pattern-dependent reaction and depletion caused by the fact that the patterns and pattern density of microstructures on the wafer vary with position.
Lastly, rapid materials and process experimentation is not achieved. Although three zones in the showerhead may allow better control of uniformity, substantial experimentation is still required, and the information contained from varying the relationship of the three zones will not substantially accelerate process learning (in the analysis of both real time sensing and post processing metrology data). Only three zones are involved and the interzone mixing affects the information content in a way which depends on the unknown process chemistry.
The shortened time scales for products in these industries demand more rapid process and product development. In an environment of new materials and processes, this presents a major difficulty, because much experimentation is required, and little fundamental knowledge exists to guide the materials and process development activities. No matter how efficient the design of such experiments may be, the complexity of the new materials combinations to be considered places a heavy burden on comprehensive experimentation which is costly and time-consuming. And even for conventional materials and processes, significant experimentation is required both in development and in manufacturing in order to optimize individual processes for materials performance and manufacturing uniformity, and to adjust the design points for several processes to a system-level optimum as required for process integration and yield.
A fundamental limitation in known conventional experimentation, both in development and in manufacturing, is that many wafers must be processed to acquire an adequate picture of materials and process performance. With single-wafer processing already a dominant trend, industry has begun to show great interest in the development and deployment of advanced process control methods which can assure wafer-to-wafer repeatability in manufacturing. Given this concern, it is clear that sequential processing of multiple wafers incurs inaccuracies associated with wafer-to-wafer variation of process and equipment, presenting a further obstacle to rapid experimentation. The demand for new, more complex materials and processes further exacerbates this problem, but at the same time it opens the door to thinking about strategies for major improvement. One example is that of combinatorial methods, in which many versions of a material are produced in parallel, with gradients of stoichiometry intentionally created across an array of samples. Additionally, few solutions have been proposed to measure uniformity through in-situ and/or real-time sensors, and none for CVD.
It is, therefore, apparent that there is a substantial need in the art to achieve a substantially higher degree of control of process uniformity and to accelerate the process development and optimization cycle by minimizing the experimentation required.
It is, therefore, an object of the present invention to produce highly controlled spatial distributions of impinging gas fluxes for CVD, plasma and other processes in microelectronics manufacturing equipment. It is another object to enable process uniformity across the wafer/substrate over a broad range of desired process design points, thereby achieving compatible co-optimization of both materials and manufacturing performance. It is yet another object of the present invention to achieve accelerated experimentation and process development by enabling controlled nonuniformity across the wafer/substrate, so that combinatorial methods provide information on multiple experimental design points in each actual experiment on a single wafer. It is a further object to facilitate sensing by gas sampling and installation of other wafer and process state sensors directly above the wafer. It is still a further object to enable the modular design of process gas delivery showerhead devices. It is another object of the present invention to provide each segment of the showerhead with both a gas inlet and a gas exhaust capability that significantly minimizes intersegment mixing. These and other objects of the present invention are realized by a multizone programmable showerhead and method for use in microelectronics processing that allows gas flow rates and compositions to be independently controlled with in each zone or segment without any significant intersegment mixing of gases. In preferred embodiments, each segment is provided with both a gas inlet and a gas exhaust capability that significantly minimizes intersegment mixing. Further preferred embodiments include modular selectively connected showerhead segments and real time gas and optical sampling mechanism associated with each segment which permit collection of real time data concerning processing.